Silanes have been found useful for surface treatment of metal surfaces such as to provide corrosion protection thereof and/or to promote adhesion of rubber thereto. These silanes can be divided into two groups in terms of their hydrophobicity: organic solvent-based and water-based. The former group is highly immiscible with water such that a large amount of organic solvent is required when preparing silane solutions.
Concerning water-based silanes, some of the major advantages are summarized as follows. (1) Water-based silanes are free of Volatile Organic Compounds (VOC) since only de-ionized water is needed for silane solution preparation. (2) There is substantial hydrolysis. Consequently, less time is required to prepare water-based silane solutions than solvent-based silane solutions. (3) Water-based silanes offer corrosion protection performance when used as a pre-treatment prior to topcoating.
Despite the advantages, one major disadvantage with current water-based silanes, for example, is that they do not provide the level of corrosion protection for metals, like the organic solvent-based silanes, when used as a surface finish. This shortcoming is believed to stem from (1) water-based silanes' intrinsic poor film coverage to metal oxides and (2) hydrophilic groups in the as-formed silane films. Accordingly, it would be desirable to provide improved silane coating compositions, which may include water-based silanes, for treatment of metal surfaces, such as to slow the rate of corrosion thereof.
In addition to using silane coating composition for corrosion prevention, adhesive bonding between metals and rubber is also of interest. By way of example, certain automobile components, e.g. tire cords and vibration dampers, rely on adhesive bonding between a metal surface and rubber. Steel tire cords, for example, are typically coated with a thin layer of brass to promote adhesion between the underlying steel and sulfur-cured rubber. In addition, adhesion promoters such as cobalt salt additives, and hexamethyl-methoxymelamine (HMMM)/resorcinol systems are used to enhance rubber adhesion to tire cords. Organic solvent-based adhesive systems also may be used in certain applications for bonding metals to sulfur-cured rubbers. Although the performance of the various methods currently employed is adequate, they still suffer from several drawbacks. Cobalt salts, for example, are expensive and pose availability problems, while brass stimulates galvanic corrosion of steel. Finally, organic solvent-based adhesives are VOC and flammable, hence, can be hazardous.
Additionally, although certain silanes, such as organic solvent-based systems, have been found to promote adhesion between a metal surface and a polymer layer, the use of organic solvents, as stated above, can be hazardous and pose concerns. The adhesive results of these silanes also are typically system dependent. In other words, the amount of adhesion provided by a particular silane coating typically depends on the metal surface as well as the polymer layer to be adhered thereto. For example, while certain silane solutions have improved adhesion between a metal surface and a peroxide-cured rubber, these same silane solutions may not necessarily provide the same results for sulfur-cured rubber. As such, it would be desirable to provide improved silane coating compositions, which include water-based silanes, for surface treatment of metal surfaces to promote adhesion of rubber, particularly sulfur-cured rubber, thereto.
Accordingly, the present invention provides for improved silane coating compositions, including water-based and organic solvent-based silane coating compositions, which include small amounts of polymeric resins, such coating compositions may be applied on metal surfaces, such as steel or aluminum alloy, for slowing their rate of corrosion and/or for bonding of rubber, e.g. sulfur-cured rubber, thereto.